CuSeO4 and Cu(SeO3OH)2·6H2O, two novel copper–selenium(VI) oxysalts

Abstract

The two new copper(II) salts Cu(SeO4) and Cu(SeO3OH)2·6H2O were synthesized at low-temperature hydrothermal conditions (220 °C), and room temperature, respectively. Their atomic arrangements were studied based on single-crystal X-ray investigations [P21/n, a = 4.823(1), b = 8.957(2), c = 6.953(1) Å, β = 94.82(1)°, Z = 4; $$P{\overline 1}$$ , a = 6.133(1), b = 6.303(1), c = 8.648(2) Å, α = 70.45(1), β = 84.60(1), γ = 73.44(1)°, Z = 1]. Cu(SeO4) adopts the MnAsO4 structure type. It exhibits structural as well as topological relations with two formerly known isochemical compounds. They crystallize in the structure type ZnSO4 (mineral name zincosite, Pnma) respectively NiSO4 (Cmcm). The two minerals dravertite, CuMg(SO4)2, and hermannjahnite, CuZn(SO4)2, are isotypic with CuSeO4-P21/n; interestingly, also α-NaCu(PO4) belongs to this structure type: some rotation of the XO4 group allows a supplementary position for the Na atom. — Cu(SeO3OH)2·6H2O represents a new structure type. The protonated selenate group shows an extended Se—Oh bond distance (1.695 Å) as compared to the other Se—O bonds (1.614 to 1.626 Å). One OH dipole of the three independent H2O molecules represents a rather free hydrogen bond. For the other H atoms, the O—H···O lengths vary from 2.585 to 2.799 Å. Interestingly, the distance Ow7···Ow7 of only 2.791 Å does not represent an edge in a coordination polyhedron and it is not preliminary involved in the hydrogen bond scheme. All Cu2+ ions in the two title compounds are in a pronounced [4 + 2] coordination. The Cu2+[4+2] atoms in Cu(SeO4) are linked to chains along [100]; in Cu(SeO3OH)2·6H2O they are not connected among each other.